[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 052053
TCDAT2

Tropical Storm Philippe Discussion Number  50
NWS National Hurricane Center Miami FL       AL172023
500 PM AST Thu Oct 05 2023

Philippe's convective structure is gradually evolving as it begins
to run into an old frontal zone to its north.  A band of rain is
developing over the northern part of the circulation along the
old front and is beginning to move over Bermuda, while other deep
convection extends east and southeast of the center.  The initial
intensity is held at 45 kt based on this morning's aircraft
reconnaissance data.  Another mission is scheduled into Philippe
this evening.

The current motion is a little faster toward the north, or 360/12
kt.  The track forecast reasoning has not changed.  Philippe should
continue accelerating northward over the western Atlantic during
the next 60 hours while moving between a mid-level high over the
central Atlantic and a deep-layer trough off the southeastern U.S.
coast.  On this track, the center will approach the coasts of Nova
Scotia, New Brunswick, and eastern Maine Saturday night.  After
that time, Philippe is expected to turn north-northwestward ahead
of an approaching mid-latitude trough, and it is forecast to become
absorbed by another area of low pressure over eastern Quebec late
Sunday.  The NHC track forecast generally lies close to the TVCA
and HCCA consensus aids during the entire forecast period.

Diagnoses from the SHIPS model suggest that southwesterly shear
will increase to 30 kt or more over the next 12-24 hours.  Because
of that, no intensification is anticipated in the short term.
Extratropical transition is likely to begin on Friday as Philippe
moves into and tightens the thickness gradient to its north, and
that process could cause the storm to strengthen a little due to
baroclinic influences.  This scenario is supported by the GFS and
ECMWF global models, as well as LGEM, IVCN, and HCCA consensus aids.
Extratropical transition is forecast to be complete by 48 hours
(Saturday afternoon) as Philippe becomes fully frontal, but the
intensity is unlikely to change much before the center reaches land.
Weakening is forecast once Philippe moves inland, and it's likely
that the strongest winds from the system will occur on the eastern
side of the circulation, primarily over parts of Atlantic Canada.

Based on the forecast for Philippe to become post-tropical by
Saturday, and after coordination with National Weather Service
offices in New England and the Canadian Hurricane Centre, the
current plan is for hazards in New England and Atlantic Canada to
be handled via local non-tropical statements and not issue tropical
watches or warnings.  We will continually assess this plan if the
forecast evolves and tropical watches or warnings become warranted.


KEY MESSAGES:

1. Tropical storm conditions are expected on Bermuda beginning
early Friday morning, and a Tropical Storm Warning remains in
effect.  Heavy rainfall is expected to affect the island today into
early Friday.  This could produce flash flooding.

2. Philippe is expected to move over portions of Atlantic Canada and
New England as a post-tropical cyclone this weekend.  Regardless of
Philippe's intensity or structure, interests in those areas should
be prepared for the possibility of strong winds and heavy rainfall
and monitor statements from their local weather office.  The
rainfall could produce isolated to scattered instances of urban and
flash flooding.


FORECAST POSITIONS AND MAX WINDS

INIT  05/2100Z 27.0N  66.2W   45 KT  50 MPH
 12H  06/0600Z 29.2N  65.8W   45 KT  50 MPH
 24H  06/1800Z 32.5N  65.7W   45 KT  50 MPH
 36H  07/0600Z 35.5N  66.1W   50 KT  60 MPH
 48H  07/1800Z 39.0N  66.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  08/0600Z 43.8N  66.7W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  08/1800Z 48.6N  71.4W   40 KT  45 MPH...POST-TROP/INLAND
 96H  09/1800Z...DISSIPATED

$$
Forecaster Berg
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